The present invention relates generally to determining the location of wireless mobile communication devices. More particularly, it relates to techniques for wireless location in an integrated system.
One way to determine position or location of an object is through the use of Global Positioning Systems (GPS). Global positioning systems, which are satellite based, provide accurate, three dimensional position information to worldwide users. FIG. 1a depicts a global positioning system (GPS) 10. The GPS 10 comprises a plurality of satellites 12-j and at least one GPS receiver 14, where j=1, 2, . . . , n. Each satellite 12-j orbits the earth at a known speed vj and is separated by a known distance from the other satellites 12-j. Each satellite 12-j transmits a global position signal 11-j which includes a carrier signal with a known frequency f modulated with a unique pseudo-random noise (PN-j) code and navigational data (ND-j) associated with the particular satellite 12-j. The PN-j code includes a unique sequence of PN chips and navigation data ND-j which includes a satellite identifier, timing information and orbital data, such as elevation angle xcex1j and azimuth angle xcfx86j. FIG. 1b depicts a typical 20 ms frame of the GPS signal 11-j which comprises twenty full sequences of a PN-j code in addition to a sequence of navigation data ND-j.
GPS receiver 14 comprises an antenna 15 for receiving GPS signals 1-j, a plurality of correlators 16-k for detecting GPS signals 11-j and a processor 17 having software for determining a position using the navigation data ND-j, where k=1, 2, . . . , m. GPS receiver 14 detects GPS signals 11-j via PN-j codes. Detecting GPS signals 11-j involves a correlation process wherein correlators 16-k are used to search for PN-j codes in a carrier frequency dimension and a code phase dimension. Such a correlation process is implemented as a real-time multiplication of phase shifted replicated PN-j codes modulated onto a replicated carrier signal with the received GPS signals 11-j, followed by an integration and dump process.
In the carrier frequency dimension, GPS receiver 14 replicates carrier signals to match the frequencies of the GPS signals 11-j as they arrive at GPS receiver 14. However, due to the Doppler effect, the frequency f at which GPS signals 11-j are transmitted changes an unknown amount xcex94fj before the signal 11-j arrives at the GPS receiver 14. Thus, each GPS signal 11-j will have a frequency f+xcex94fj when it arrives at the GPS receiver 14. To compensate for the Doppler effect, GPS receiver 14 replicates the carrier signals across a frequency spectrum fspec ranging from f+xcex94fmin to f+xcex94fmax until the frequency of the replicated carrier signal matches the frequency of the received GPS signal 11-j, wherein xcex94fmin and xcex94fmax are a minimum and maximum change in the frequency the GPS signals 11-j will undergo due to the Doppler effect as they travel from satellites 12-j to GPS receiver 14, i.e., xcex94fminxe2x89xa6xcex94fj xe2x89xa6xcex94fmax.
In the code phase dimension, GPS receiver 14 replicates the unique PN-j codes associated with each satellite 12-j. The phases of the replicated PN-j codes are shifted across code phase spectrums Rj,(spec) until the replicated carrier signals modulated with the replicated PN-j codes correlate, if at all, with the GPS signals 11-j being received by the GPS receiver 14, where each code phase spectrum Rj(spec) includes every possible phase shift for the associated PN-j code. When the GPS signals 11-j are detected by the correlators 16-k, GPS receiver 14 extracts the navigation data ND-j from the detected GPS signals 11-j and uses the navigation data ND-j to determine a location for the GPS receiver 14.
A GPS enables a ground based receiver to determine its position by measuring the time difference required for GPS signals initiated from two or more satellites to be received by the GPS receiver 14. The pseudorange is defined as this time difference times the speed of light. The pseudorange is not the real range because it contains errors caused by the receiver clock offset. To determine a two-dimensional position (latitude and longitude) usually entails receiving signals from at least three satellites. To determine a three-dimensional position (latitude, longitude, and altitude) usually entails requires receiving pseudoranges from four or more satellites. This precondition, however, may not always be satisfied, especially when the direct satellite signals are obstructed, such as when a wireless terminal is inside a building.
GPS receivers, such as GPS receiver 14, are now being incorporated into wireless mobile communication devices (including mobile telephones, PDAs, pagers, portable computers, etc.). However, these devices do not always have a clear view of the sky. In this situation, the signal-to-noise ratios of GPS signals 11-j received by GPS receiver 14 are typically much lower than when GPS receiver 14 does have a clear view of the sky, thus making it more difficult for GPS receiver 14 to detect the GPS signals 11-j.
Integrated wireless-global positioning (WGP) systems were developed to facilitate the detection of GPS signals by GPS receivers in wireless mobile communication devices. The WGP system facilitates detection of GPS signals by reducing the number of integrations to be performed by correlators searching for GPS signals. The number of integrations is reduced by narrowing the frequency range and code phase ranges to be searched. Specifically, the WGP system limits the search for GPS signals to a specific frequency or frequencies and to a range of code phases less than the code phase spectrum Rj,(spec). However, problems of obstructed signals still exist. Further, an even greater problem involves the fact that many wireless mobile communication devices do not include any type of GPS receiver. These type of xe2x80x9clegacyxe2x80x9d devices, therefore, need to be located by some other measure.
Another known way of determining the position of a wireless mobile communication device is to utilize information obtained from a wireless network. One such method of geolocation uses signal strength measurements. In the IS136 and IS54 standards, signal strengths are extensively used for so-called MAHO (mobile-assisted handoff) process. The MAHO measurement contains signal strength information, which reflects the distance between the wireless terminal and a base station (BS). MAHO measurement lists are routinely delivered by wireless mobile communication devices for handoff purposes and form the basis of a low-accuracy geolocation system based on either, or a combination, of two techniques: xe2x80x9ctriangulationxe2x80x9d and xe2x80x9ccontour matchingxe2x80x9d.
In the first technique, xe2x80x9ctriangulation,xe2x80x9d the signal strength from multiple MAHO channels is associated with a location of a wireless mobile communication device. This then produces a geometric triangulation mathematical problem that can be solved to determine the location of the wireless mobile communication device (xe2x80x9cwireless mobilexe2x80x9d). FIG. 2 illustrates a known method for determining a location from which a mobile caller originates a call on a wireless mobile 102. Specifically, a signal originating from at least one base station, such as base station 104, reaches the wireless mobile 102 with a particular signal strength at a particular time. Similarly, signals from base stations 106 and 108 send similar signals, which arrive at wireless mobile 102 at the same time but with varying signal strengths. In aIS136-based network, many such signals arrive at a given instant and their strengths are recorded for handoff purposes. Methods for using these data to determine such a location of the wireless mobile 102 are well-known and will not being further described for the sake of brevity.
In the second technique, termed xe2x80x9ccontour matchingxe2x80x9d, the wireless system receives MAHO measurements and compares these relative signal strength measurements to a specifically-developed database of stored positions (grid locations) and relative signal strength measurements within the cell serving the call. The wireless system can then determine the location of the wireless mobile communication device by matching its signal strength measurements to the corresponding signal strength measurements of one of these grid locations in the database. An improvement on the grid system was to establish contours between grid points using interpolation and in some cases prediction from simulations or propagation models. Once the database is established, received signals can be matched to the contours, thus the name xe2x80x9ccontour matching.xe2x80x9d
An alternative to the MAHO technique discussed is the so-called Enhanced Cell Global Identity (ECGI) method. CGI is a standardized technique for geolocation in the Global System for Mobile communications (GSM), and involves enhancing CGI by database matching techniques described previously. For GSM, however, what we have called MAHO is replaced by Timing Advance (TA). TA measurements, like MAHO measurements, are made at the mobile terminal (wireless mobile communication device) and can determine, albeit with low precision, the location of a wireless mobile communication device. TA measurements differ fundamentally from MAHO measurements in that TA is time-based and MAHO is strength-based, but both can be used as indicators of distance, and thus be applied to location finding.
To summarize concepts begun above, a database technique can be used to locate wireless mobile communication devices. The database technique can use direct (grid based) data matching or a contour matching technique. The underlying data populating the database always contains geographic coordinates (e.g., latitude/longitude data), and also may contain either signal strength data (such as MAHO data), TA data, both or other data types. Other data types may be used as well.
However, a major problem with using wireless network based signals such as signal strength (MAHO) data, TA data, etc., to determine the location of a wireless mobile communication device is that measurement errors are nearly always much larger than the satellite based navigational system measurement errors. Further, there are many problems involved with creating the database. Signals may be blocked, drive testing to each location in each cell may be tedious and impractical, small samples from a cell are typically used, etc. Additionally, when network operational parameters change or new basestations are brought on line, the database must be recalibrated.
Integrated wireless-global positioning (WGP) systems rely on both the satellite navigation system and the wireless communication system to determine the location of a wireless terminal. An integrated wireless-global positioning system combines the data from both the wireless network and the satellite navigation system to obtain an integrated position solution. By combining information from both the global positioning system and the wireless network, it is possible to increase the positioning accuracy and, at the same time, overcome the requirement of having at least three measurements from a single source type. However, such systems do not exist in all currently used wireless mobile communication devices, and thus other methods must be used in connection with existing legacy wireless mobile communication devices.
FIG. 3 depicts an integrated wireless global positioning system 20 comprising a WGP server 22, a plurality of base stations 23 and at least one WGP client 24. WGP server 22 includes a GPS receiver 26 having an antenna 27 installed in a known stationary location with a clear view of the sky. WGP server 22 is operable to communicate with base stations 23 either via a wired or wireless interface. Each base station 23 is at a known location and provides communication services to WGP clients located within a geographical area or cell 25 associated with the base station 23, wherein each cell 25 is a known size and may be divided into a plurality of sectors. WGP client 24 includes a GPS receiver 28 and perhaps a wireless terminal such as a wireless telephone 29, and is typically in motion and/or at an unknown location with or without a clear view of the sky.
FIG. 4 is a flowchart 300 illustrating the operation of WGP system 20. In step 310, WGP server 22 detects a plurality of satellites 12-j via their GPS signals 11-j using its GPS receiver 26. WGP server 22 acquires the following information from each detected satellite 12-j: the identity of satellite 12-j and frequency fj, code phase, elevation angle xcex1j and azimuth angle xcfx86j associated with the detected satellite 12-j, wherein the elevation angle xcex1j is defined as the angle between the line of sight from WGP server 22 or client 24 to a satellite 12-j and a projection of the line of sight on the horizontal plane, and the azimuth angle xcfx86j is defined as the angle between the projection of the line of sight on the horizontal plane and a projection of the north direction on the horizontal plane. FIG. 5 depicts an elevation angle xcex1j and an azimuth angle xcfx86j corresponding to a satellite 12-j and a WGP server 22 or WGP client 24.
In step 315, WGP server 22 receives sector information from base station 23 currently in communication with or serving WGP client 24, wherein the sector information indicates the sector WGP client 24 is currently located. In step 320, WGP server 22 makes an initial estimate of WGP client""s position based on the known location of the serving base station, the cell size associated with the serving base station, and the sector in which WGP client 24 is currently located. In one embodiment, WGP server 22 initially estimates that WGP client 24 is located at a reference point within the sector, e.g., point at approximate center of sector. In another embodiment, WGP server 22 initially estimates WGP client 24""s position using known forward link triangulation techniques.
In step 330, for each detected satellite 12-j, WGP server 22 uses the information acquired from the detected GPS signals 11-j to predict a frequency fj(r) at the reference point and a code phase search range Rj(sect) which includes all possible code phases for GPS signal 11-j arriving anywhere within the sector where WGP client 24 is currently located. In step 340, WGP server 22 transmits a search message to the serving base station 23, where the search message includes, for each detected satellite 12-j, information regarding the associated PN-j code, predicted frequency fj(r) and code phase search range Rj(sect).
In step 350, serving base station 23 transmits the search message to WGP client 24 which, in step 360, begins a parallel search for the satellites 12-j indicated in the search message. Specifically, WGP client 24 uses its correlators to simultaneously search for each of the GPS signals 11-j at the predicted frequency fj(r) within the limitations of the code phase search range Rj(sect) indicated in the search message.
The performance of an integrated wireless-global positioning system depends directly on the quality and accuracy of the information received from the satellite and the wireless network, namely, the pseudoranges and the signal strength (MAHO), TA data, etc. Unfortunately, these measurements are normally noisy and especially for the technique referred to above as xe2x80x9ccontour matching,xe2x80x9d it is difficult to initially build and subsequently maintain a proper database. Accurately determining the required database normally requires extensive xe2x80x9cdrive testingxe2x80x9d, where calibrated equipment is driven around a wireless coverage area making the necessary measurements in a calibrated fashion to help populate the database. Drive testing may have to be repeated periodically. This labor-intensive process can render the technique too costly.
Accordingly, a need exists for determining the position of wireless mobile communication devices which do not have unobstructed reception of the satellite signals, are not equipped with GPS capabilities. This latter category, wireless mobile communication devices without GPS capability, is crucial because during a transition period many wireless mobiles will lack such capability. During the transition period, which may last almost indefinitely, legacy mobiles will still need to be located for emergency services, such as E911 calls, for example.
A system and method of the present application involves calibration or population of a position locating system, with reliable position information (from GPS signals, position signals derived from enhanced based position techniques, etc.). Thereafter, both downlink network information and reliable position information are associated together. Thus, network downlink information (MAHO data for signal strength/TA data, etc.) is associated with accurate position information to accurately populate a database, and can thereafter be used to determine position based on triangulation, contour matching, etc. Such an accurate location system can thereafter be used to locate a wireless mobile communication device, such as in E911 situations, for example.